Abstract: This paper briefly describes the importance of current stabilization in rectifier devices and the application of automatic current stabilization devices in rectifier systems. The structure, performance, computer control technology, and negative feedback closed-loop control system of the automatic current stabilization device are introduced. Keywords: Rectification; Automatic current stabilization; Rectifier transformer; Thyristor rectifier cabinet; Digital trigger regulation negative feedback [b]1 Introduction to Automatic Current Stabilization Systems[/b] Since the 1990s, with the widespread application of high-power thyristor rectifier devices and the successive commissioning of ion-exchange membrane electrolysis devices, automatic current stabilization devices for automatically adjusting DC output current have gradually matured. In chlor-alkali production, fluctuations in DC current, high-voltage power grid, and changes in the number of electrolytic cells directly affect the stable operation of the electrolytic cells. Unstable DC current output also affects current efficiency, leading to increased unit consumption of electrolytic products. Thyristor automatic current stabilization devices can effectively solve this problem, providing stable DC current output and achieving precise current stabilization together with computer automatic control technology. Currently, there are three types of automatic current stabilization systems that use the output current of high-power rectifier equipment as the adjustment target: one is ampere. There are three types of automatic current stabilization systems: time-based, proportional, and computer-based. Time-based automatic current stabilization systems are adjustable systems that can obtain high-precision average current values ​​over long periods. However, these systems have slow response speeds and high inertia, making them unsuitable for use in electrochemical industrial rectifiers. Proportional automatic current stabilization systems are adjustable systems that respond to instantaneous current changes, with low inertia and fast response speeds. These systems are well-suited for electrochemical processes. Additionally, new computer-based automatic current stabilization systems utilize computer automatic control principles to stabilize the current. Closed-loop negative feedback achieves precise control of the output current through internal CPU logic operations. [b]2 Structure of the Thyristor Current Stabilizer[/b] 2.1 Composition of the Thyristor Current Stabilizer The thyristor current stabilizer system consists of a rectifier transformer, a thyristor rectifier, a polarization power supply, and a dual-channel computer control cabinet. During the construction of the Wu Hua Aerospace Chemical Co., Ltd. project... Electrolytic cells require phased and batch-based commissioning. The varying number of cells entering production necessitates different output currents and voltages from the rectifier unit. When using a thyristor-based direct-fallback rectifier unit, the initial number of electrolytic cells requires a low DC voltage, placing the rectifier unit in a deep control state. At this point, the thyristor's control angle is very large, resulting in high harmonic currents, severe distortion of the grid voltage waveform, a very low power factor, and significant vibration of the rectifier unit – these are the main risks associated with using thyristor rectifiers. However, a rectifier unit combining an on-load tap-changing rectifier transformer and a thyristor rectifier allows for single-unit DC current control by the thyristor, enabling continuous stepless current regulation. By adjusting the transformer's voltage regulator to an appropriate setting, the AC voltage of the rectifier unit is reduced, allowing the thyristor rectifier unit to operate with a control angle close to zero. In this state, the rectifier unit's operation is essentially the same as that of a diode rectifier unit, thus avoiding the risks associated with using thyristors. 2.2 Rectifier Transformers The rectifier transformers are ZHSFPZ-13600/35 type 27-pole on-load tap-changing transformers, with one transformer supporting two rectifier cabinets. Both rectifier transformers use autotransformer neutral point tap-changing connections. The valve-side (secondary) windings have two sets of positive and negative angle connections. The grid-side rated voltage is 35kV, the valve-side DC voltage is 430V, the impedance voltage is 8%, and the equivalent pulse p=6. The two rectifier transformers have grid-side electrical angles of 7.5° and 22.5°, differing by 15°, forming a single unit with 12 pulses. The two units constitute an equivalent 24-phase rectification, effectively suppressing harmonics below the 23rd order and improving the system power quality. The rectifier depot currently has four rectifier transformers supporting eight rectifier units. To avoid a decrease in power factor due to thyristor rectification, a combination of on-load tap changer coarse adjustment and trigger angle control for fine adjustment is adopted. When the conduction angle is less than the set value for upgrading, the host computer prompts for upgrading, avoiding frequent switching of the on-load tap changer due to automatic conduction angle linkage, which would reduce the lifespan of the switch. 2.3 Rectifier Cabinet The rectifier cabinet adopts the KHS-13500A/430V type, and the second phase adopts the KHs_18000A/430V type, with DC rated currents of 13.5kA and 18kA respectively. It is a three-phase bridge type with in-phase reverse parallel connection. The rectifier devices and protection components of the bus rectifier circuit in the cabinet adopt KP3000A/1600V type thyristors and RSG_6D-500V/3600A type fast fuses respectively. Each bridge arm uses 3 thyristors in parallel. The current and voltage margins of the rectifier cabinet are both greater than 3, the current sharing coefficient of the same arm is >10.9, and it is cooled by forced circulation of industrial water. Thyristor rectifiers have high efficiency, approximately 97%. A key characteristic of thyristors is the presence of harmonics in the input current, caused by reactive power generation due to the firing angle. Therefore, harmonic and reactive power compensation is necessary. The advantages of this rectifier frame include high efficiency, high reliability, good load current control, low cost, and mature technology. Disadvantages include reactive power generation, current harmonics, the need for filters, and current ripple. 2.4 The dual-channel computer control cabinet uses the RICS-5D model dual-channel computer control cabinet. The control system is computer-controlled, mainly consisting of a dual-channel computer triggering and current stabilization system, and a PLC monitoring and integrated protection system. Operation and monitoring are achieved on a host computer via a network, interconnecting with the DCS system. Automatic current stabilization accuracy is better than ±0.5%. Phase shift range is 0–150°. Employing redundancy technology, the system automatically switches to a hot standby channel when a fault occurs in the operating channel. The switching process is uninterrupted by current disturbance, allowing for seamless replacement of the faulty channel without power outages. The CNC operator, digital adjustment trigger, and dual-channel hot standby controller are interconnected via a simple and reliable micro-network, automatically detecting the entry, exit, and fault status of the CNC operator. The rectifier cabinet's current regulation controller uses a single-chip microcomputer-based dual-channel digital system, with the digital control and digital trigger channels serving as hot standby for each other. Both channels can operate independently and possess automatic diagnostic functions, enabling convenient and seamless switching based on operating conditions. The relay protection control core uses an S7-200 PLC programmable controller. This powerful CPU offers large node capacity and numerous expansion options. Single-cabinet PLCs are installed and controlled locally, enabling effective and reliable local control and protection. Data is then transmitted to the host computer for local/remote control. 2.5 Polarization power supply During the power failure of the main circuit of the rectifier cabinet or the shutdown and maintenance of the electrolytic cell, timely activation of the polarization power supply can effectively protect the electrolytic cell. The electrolytic cell adopts thyristor pulse blockade control, which has a fast response speed. At the same time, the tap device can be adjusted after the electrolytic cell is added or removed to reduce the harmonic hazards of the polarization system device. It forms a protection for the positive and negative electrodes and ion membrane of the ion membrane electrolytic cell, preventing the positive and negative electrodes of the ion membrane electrolytic cell from being corroded. 3 Current stabilization working principle and application 3.1 Current stabilization (1) Local setting. The required operating current is manually set on the rectifier control cabinet, which is generally used in the commissioning stage. (2) OP-05 operator setting. Jogging, continuous rise/fall, rapid rise/fall. (3) Host computer/DCS operation setting Jogging, continuous rise/fall, direct input value on the host computer in the main control room. During normal operation, the operator can flexibly control the operating current of each rectifier in the main control room. They are not constrained by each other. Practice has proven that setting the above three given values ​​allows for flexible and rapid operation. When switching between units, there is no significant fluctuation in the series current. During the computer-controlled unit switching process, the output current values ​​of each unit are automatically and evenly distributed. This reliably protects the needs of electrolytic production. 3.2 Computer Closed-Loop Feedback System To ensure the stability and reliability of the automatic current stabilization system, the operating signal acquisition adopts a dual-loop form, with both DC and AC feedback. Shielded twisted-pair cables are used for signal transmission, forming a reliable closed-loop system. The AC feedback signal is taken from the valve-side current transformer of the rectifier transformer. The DC signal is taken from the DC transformer on the DC bus. The DC transformer uses a ZY type, which is made using the Hall effect induction principle and features high precision, low loss, and accurate measurement. Under normal circumstances, the DC signal is used as the feedback signal, forming a closed-loop negative feedback system. The DC current output is adjusted by comparing the given signal from the control cabinet with the CPU's internal signal, thereby ensuring the stable operation of the current stabilization system. 3.3 Current Stabilization Process The automatic current stabilization system in each rectifier control cabinet adopts an independent small-loop current control system. It possesses superior current stabilization performance. When the system operates in automatic current stabilization mode, the CPU compares the current setpoint signal and the feedback signal, and performs PI regulation calculation on the deviation. When the grid voltage or load changes, the output current fluctuates accordingly, and the current feedback signal changes as well. After PI regulation calculation, a corresponding phase-shift control quantity is output to the trigger, causing the phase (1r angle) of the trigger's output pulse to change accordingly, thereby adjusting the DC output current and achieving the purpose of closed-loop automatic current stabilization. The current stabilization process can be briefly described as follows: 3.4 Signal Protection System of Rectifier Cabinets Each rectifier cabinet is equipped with a rectifier control panel, which has comprehensive protection functions. Each control panel is equipped with a Siemens S7-200 programmable logic controller (PLC). The PLC relay protection system has the following functions: alarms are triggered when a single fast-blow fuse blows, rectifier bridge arm overheats, or cooling water temperature is too high; pulses are blocked to reduce the DC current output to zero when power phase loss or overcurrent occurs; pulses are blocked and high-voltage tripping is performed when operational overvoltage faults, multiple fast-blow fuses blow, or emergency stop occur, reducing the DC current output to zero; an alarm is triggered first when the cooling water pressure is below 0.1 MPa, and if the fault is not cleared within 9 seconds, pulses are blocked and high-voltage tripping is performed, reducing the DC current output to zero. The block diagram of the automatic current stabilization system of the thyristor rectifier is shown in Figure 1. As can be seen from Figure 1, the digital adjustment trigger controller is the core of the current stabilization system. The programmable logic controller is the foundation for the operation and protection of the current stabilization system. 4 Conclusion The thyristor current stabilization device at Wuhua Aerospace Chemical Co., Ltd. has been operating stably since its commissioning. During the full-load test in October 2004, the rectified current was 9800A. Through automated monitoring and on-site testing, the rectified current fluctuation range was within +50A, and the current stabilization accuracy was ≤±0.5%, providing highly stable DC current and ensuring the safe production of the electrolytic cells. In summary, with the continuous development and improvement of computer technology and thyristor rectification technology, using a computer as the data management and current stabilization control device for rectifiers can achieve excellent current stabilization effects and significantly improve converter efficiency. The thyristor rectifier equipment used by the company has been operating for three years. The overall system performance is excellent, and the technical indicators such as stability accuracy, current sharing coefficient, and harmonic management have all met the design requirements, effectively ensuring the safe and stable operation of the rectifier. Click to download: Application of Thyristor Current Stabilization Device in Rectifier Systems. Editor: Chen Dong